Call set-up in a transmission trunking radio system

ABSTRACT

A call control method in a transmission trunking radio system, in which (i) M traffic channels are simultaneously allocated to at most N mobile stations for a call, N and M being positive integers and N&gt;M; and (ii) a mobile station is commanded to go onto one of the M traffic channels during a call for the duration of the call transactions only and onto a control channel at other times. The number N of calls can be optimized by (iii) collecting data about collisions of call transactions of different calls on the M traffic channels, and (iv) automatically adjusting the maximum number N of simultaneously allowed calls in accordance with the collected data.

FIELD OF THE INVENTION

The invention relates to a call control method in a transmissiontrunking radio system.

BACKGROUND OF THE INVENTION

In a trunking radio system, several user groups, even several userorganizations, share a common pool of channels. In radio systems likethis, a message trunking procedure is usually utilized for call set-up.In this procedure a traffic channel is permanently allocated for onecall for the entire duration of the call. The call may consist ofvarious separate call transactions (several pressel activations byseparate terminals). The channel is not de-allocated (released) untilthe call is explicitly terminated or a specific timer expires.

In a transmission trunking procedure, a group of simultaneous callsshares a certain group of traffic channels; for each call, a trafficchannel belonging to this group is allocated, separately for each calltransaction (each activation of the pressel). The traffic channel isimmediately de-allocated at the end of the call transaction, i.e. whenthe pressel is released and one stops speaking. Between the calltransactions, the MS participating in the call goes to listen onto acontrol channel. It is typical of transmission trunking that allocationof a channel requires only a little signalling during an on-going call.

On account of transmission trunking, for example, more than four callscan, in principle, be allowed simultaneously in a 4-channel system.However, there is no guarantee that a subscriber can be served at theprecise moment the service is requested for. Another problem withoptimisation of channel utilization is that it is not necessarilypossible to make a reliable estimate of the number of calls that can beallowed simultaneously on transmission trunking channels since theamount of traffic may vary from day to day. For example, a very largenumber of calls by which subscribers give brief instructions to oneanother relatively seldom (e.g. group call) can be allowedsimultaneously. On the other hand, one call with normal conversationoccupies an entire traffic channel for a long time.

SUMMARY OF THE INVENTION

An object of the present invention is to optimise the number ofsimultaneous calls in a transmission trunking radio system.

This is achieved according to the invention with a call control methodin a transmission trunking radio system, in which M traffic channels aresimultaneously allocated to at most N mobile stations for a call, N andM being positive integers and N>M; and during a call a mobile station iscommanded to go onto one of the M traffic channels for the duration ofthe call transactions only and to go onto a control channel at othertimes. The method is characterised by collecting data about collisionsof call transactions of different calls on said M traffic channels, andautomatically adjusting the maximum number N of simultaneously allowablecalls in accordance with the collected data.

The invention offers a solution to the problem of optimising the numberof calls: the radio system collects collision data about calltransactions of different calls, e.g. at a base station or a mobileexchange. On the basis of these traffic and collision data the radiosystem automatically controls the number of simultaneous calls,increasing the number if no collisions occur and reducing it on certainconditions if collisions occur. It is thus possible to establish severalsimultaneous calls if traffic is such that the probability of collisionis small.

A requirement for collection of collision data is that it is possible tomake a relatively reliable estimate of an occurrence of a collision. Inone embodiment of the invention, a mobile station sends a channelrequest to a base station for each service transaction. Each situationwhere the system is not capable of immediately allocating a trafficchannel because no free channels are available is then interpreted as acollision of call transactions.

The maximum number of simultaneous calls can be adaptively adjusted onthe basis of the number and frequency of collisions. The adaptation maydepend e.g. on the ratio of such channel allocation attempts which havecaused collisions to the total number of allocation attempts in a timeunit.

The invention also relates to a transmission trunking radio systemcomprising base station sites (BS), mobile stations (MS), call controlmeans (MX) and, in at least one base station site, at least one controlchannel and at least M traffic channels in which at most N simultaneouscalls are allowed, N and M being positive integers and N>M; the callcontrol means (MX) commanding the mobile station to go onto one of the Mtraffic channels during a call for the duration of the call transactionsand onto a control channel at other times. The system of the inventionis characterized in that it further comprises means (MX) for collectingdata about collisions of call transactions of different calls on said Mtraffic channels, and means (MX) for automatically adjusting the maximumnumber N of simultaneously allowable calls in accordance with thecollected data.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the preferred embodiments of the invention will bedescribed in greater detail with reference to the attached drawings, inwhich

FIG. 1 illustrates a radio system in which the invention may be applied,

FIG. 2 shows a general block diagram of a mobile station,

FIG. 3 shows a signalling scheme illustrating a transmission trunkingprocedure, and

FIG. 4 is a flow diagram of one optimisation algorithm according to theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a trunking radio telephone system in which the geographicalarea covered by the system is divided into smaller radio areas or cellsC1, C2 and C3, which may be separate, defined by one another or overlapin their peripheral areas. Each cell C1, C2, C3 contains at least onefixed--typically multichannel--transceiver apparatus BS, which is calleda base station. All base stations BS are connected by fixed transmissionlinks, such as cables, to a mobile exchange MX, which controls theoperation of the base stations BS. The base stations BS communicate viaa radio connection with subscriber mobile stations MS roaming freelywithin the radio system on the radio frequencies or channels assigned tothe radio system.

A trunking radio telephone system may be a digital TDMA system or ananalog FDMA system. The analog FDMA system may be e.g. of the typedescribed in Finnish Patent Application 914,654. In an analog FDMAsystem each radio channel serves as a traffic channel or a controlchannel. A control channel is a channel on which signalling forcontrolling the system is transmitted. A traffic channel is a channel onwhich speech or data are transmitted. In a TDMA system each radiochannel is further divided into timeslots (e.g. 4 or 8 timeslots perradio channel), in which control and traffic channels are conveyed.

In radio systems, a so called message trunking principle is normallyused for call set-up: a traffic channel is allocated for one call forthe complete duration of the call. The number of simultaneous calls isthus limited to the number of traffic channels.

A call is complete information exchange between two or more parties. Acall may be made up of one or more call transactions. In a semi-duplexcall these transactions are sequential. A call transaction refersgenerally to all of the functions associated with completeunidirectional transmission of information during a call.

The invention relates to a trunking radio system in which a so calledtransmission trunking procedure is utilized at least on some trafficchannels. The number of simultaneous calls may exceed the number oftraffic channels available, a traffic channel being allocated separatelyfor each call transaction (each activation of the pressel). A trafficchannel is deallocated immediately at the end of the call transaction,i.e. as the pressel is released and one stops speaking. During theinterval between call transactions a mobile station participating in thecall goes onto a control channel. One application of this is known asquasi-transmission trunking, which is otherwise similar to conventionaltransmission trunking but channel deallocation is delayed for a shortperiod at the end of the call transaction (after the pressel release).During this `hang-time` the channel allocation may be re-used for a newcall transaction that is part of the same call.

In the following the transmission trunking principle will be describedwith reference to FIG. 3. In this embodiment it is assumed that a callis established between two mobile stations MSA and MSB located withinthe area of one base station. The MSA and MSB listen on a controlchannel, ready to go, if necessary, for the duration of calltransactions onto the M traffic channels available, M being e.g. 4. Whenthe MSB user wants to speak, he activates the pressel, whereby the MSBsends a request-to-transmit message U TX DEMAND on a control channel tothe base station BS. The base station BS forwards the message to themobile exchange MX. The MX comprises a call control computer, whichallocates free traffic channels to the calling parties and sends apermission-to-transmit message D TX GRANTED through the base station BSto the MSB, and a D CONTINUE message to the other calling party MSA,thereby announcing the call transaction to be performed. Each messagealso contains channel allocation, which commands the mobile stations MSAand MSB to the traffic channels allocated to them. The MSA and MSB thengo onto the traffic channels allocated, and the MSB initiatestransmission of speech through the base station BS to the mobile stationMSA. When the MSB subscriber terminates the call transaction, hereleases the pressel, whereby the MSB sends an end-of-transmissionmessage U TX CEASED to the base station BS. The call control computersubsequently de-allocates the traffic channel concerned and sends anend-of-transmission message D TX CEASED to both parties MSA and MSB,whereby MSA and MSB go back onto a control channel. One successful calltransaction I has thus been completed.

In FIG. 3, the MSB user wants to continue the call and presses thepressel, whereby the MSB sends a new request-to-transmit message U TXDEMAND. However, the call control computer of the mobile exchange MX hasno free channels available at that moment, and so the MX puts therequest in a queue and sends a D-TX WAIT message through the basestation BS to the calling parties MSA and MSB on a control channel. Whena traffic channel is then freed, the MX allocates it for a call andsends D-TX GRANTED and D-CONTINUE messages to the mobile stations MSAand MSB in the above manner. The MSB and MSA then go onto the trafficchannel allocated to them for speech transmission. When the pressel isreleased, the MSB sends an end-of-transmission message U-TX CEASED,whereby the channel is de-allocated as described in connection with calltransaction I. Another successful call transaction II in which the MSBhas had to queue for channel resources has thus been completed.

A call may be made of a plurality of call transactions of the typedescribed above, either one of the calling parties MSA and MSBinitiating said transactions. When the parties want to terminate a call,they initiate a conventional call release procedure.

As stated above, a certain number M of transmission trunking trafficchannels may be simultaneously allocated for a larger number of calls.The problem is that there is no guarantee that free traffic channels canbe found and that a subscriber can be served at the precise moment hewants to. Another problem with the optimisation of the number ofchannels is that it is not necessarily possible to make a very reliableestimate of the highest level of traffic allowed since situations mayvary from day to day depending on the users of the system. In theinvention the radio system collects collision data of call transactions;on the basis of these data the radio system then defines the maximumnumber of simultaneous calls allowable in a given situation. Thecondition for this is that the system is capable of estimating fairlyreliably when a collision has occurred. For example, in a systemfollowing the procedure of FIG. 3, in which a mobile station MS requestsa traffic channel for each call transaction, every situation in which atraffic channel cannot be allocated immediately may be regarded as acollision. For example, in FIG. 3 the call in call transaction II has toqueue for resources (traffic channel) since each traffic channel isallocated for call transactions of simultaneous calls, i.e. a collisionhas occurred.

Depending on the system, other suitable methods may also be applied todetect a collision/congestion of calls.

The system adjusts the number of simultaneous calls on the basis of thecollected traffic and collision data. If the number or frequency ofcollisions increases, the maximum number of simultaneously allowablecalls is reduced. If no collisions occur, the system increases thenumber of simultaneous calls. The maximum number of calls may alsochange with the ratio of such channel allocation attempts which havecaused collisions to the total number of allocation attempts within acertain period of time.

FIG. 4 shows an algorithm for the optimisation of the number of calls.The algorithm is always initiated when a new channel allocation attemptis made. In the embodiment of FIG. 3, this means that a U-TX DEMANDmessage is sent. First the time from the previous collision iscalculated or measured. It is then checked whether the channelallocation attempt was immediately successful or whether resources hadto be queued for on account of collision. If there is no collision, thenumber of allowable calls is integrated upward by an integrationconstant selected by the operator. In case of collision, the maximumnumber of allowable calls is integrated downward. It is then checkedthat the integration result does not go beyond certain limits set to theradio system. The limits may be due to limitations inherent in the radiosystem (e.g. the radio system may have only five bits for identificationof calls; i.e. a maximum of 32 calls), or a lower limit set by theoperator. The integration result is then used as the new maximum numberuntil the following integration. It is advantageous to the operation ofthe radio system that the control algorithm is integrative (contains acertain time constant). Without integration, e.g. when the traffic loadis low, even a large number of calls may cause few collisions or none atall, whereby the maximum number of calls allowed rises high. If theamount of traffic then increases abruptly due to increase in callactivity, the result is a large number of failed call transactions onaccount of too numerous calls.

In a fixed network, collection of data and calculation associated withoptimisation according to the invention may be included in the operationof any component of the network, such as a mobile exchange or a basestation, but most advantageously of a call control computer located inthe mobile exchange MX, the computer controlling channel allocation andall signalling associated with call control.

The mobile station MS may be of any type, depending on the radio system.FIG. 2 shows a general block diagram of subscriber equipment MS in whichthe invention may be applied. The MS comprises a transceiver 21connected to an antenna 27, the receiver being connected to aloudspeaker 25 and the transmitter being connected to a microphone 26.The operation of the MS is controlled by a microprocessor 22, whichprocesses the signals transmitted and received by the transceiver 21.The subscriber equipment MS also comprises a pressel S1, or a switch,which the speaker activates (closes) as he speaks. The microprocessor 22detects that the pressel S1 is activated and starts the above-describedoperations to perform a call transaction. On detecting the release ofthe pressel S1, the microprocessor 22 starts the above operations toterminate the call transaction.

The attached figures and the description thereof are intended only toillustrate the present invention. The call control method and radiosystem according to the invention may vary in their details within thescope and spirit of the attached claims.

I claim:
 1. A call control method in a transmission trunking radiosystem, comprising the steps of:sharing M traffic channels bysimultaneous on-going calls of N mobile stations in maximum according toa transmission trunking method, N and M being positive integers and N>M;requesting traffic channel for each call transaction by said mobilestations via a control channel during said calls; allocating to any oneof said N mobile stations requesting a traffic channel any one of said Mtraffic channels, if available, for a duration of a call transaction,and deallocating said traffic channel and commanding said one mobilestation to return onto said control channel after completion of saidcall transaction; registering collision of call transactions, if theradio system, due to a lack of available traffic channels, fails toimmediately allocate any one of said M traffic channels to any one ofsaid N mobile stations requesting a traffic channel; collecting dataabout collisions of call transactions of different calls on said Mtraffic channels; and automatically adjusting the allowed maximum numberN of simultaneous calls in accordance with the collected data.
 2. Themethod of claim 1, further comprising the steps of:initiating the calltransaction by activating a pressel in the mobile station; andterminating the call transaction by releasing the pressel in the mobilestation.
 3. The method of claim 1, further comprising the stepof:adjusting the allowed maximum number N of simultaneous calls inaccordance with the number of collisions of call transactions.
 4. Themethod of claim 3, further comprising the step of:adjusting the allowedmaximum number N of simultaneous calls in accordance with the frequencyof collisions of call transactions.
 5. The method of claim 3 furthercomprising the step of:adjusting the allowed maximum number N ofsimultaneous calls in accordance with a ratio of such channel allocationattempts which have caused collisions to the total number of allocationattempts in a time unit.
 6. The method of claim 1, further comprisingthe step of:adjusting the allowed maximum number N of simultaneous callsin accordance with the frequency of collisions of call transactions. 7.The method of claim 6, further comprising the step of:adjusting theallowed maximum number N of simultaneous calls in accordance with theratio of such channel allocation attempts which have caused collisionsto a total number of allocation attempts in a time unit.
 8. The methodof claim 1, further comprising the step of:adjusting the allowed maximumnumber N of simultaneous calls in accordance with a ratio of suchchannel allocation attempts which have caused collisions to the totalnumber of allocation attempts in a time unit.
 9. The method of claim 1,wherein said step of adjusting comprises, in connection with everychannel allocation attempt, the further steps of:measuring a timeelapsed since a previous collision; integrating the maximum number N ofsimultaneous calls downwards if the allocation attempt causes collisionof call transactions; and integrating the maximum number N ofsimultaneous calls upwards if the allocation does not cause collision ofcall transactions.
 10. Transmission trunking radio system, comprising:aplurality of base station sites; a plurality of mobile stations; atleast one of said base station sites having at least one control channeland at least M traffic channels for shared use of simultaneous on-goingcalls of N mobile stations in maximum according to a transmissiontrunking method, N and M being positive integers and N>M; said N mobilestations being arranged to request traffic channel for each calltransaction via said control channel during said calls; a call controlunit arranged to allocate to any one of said N mobile stationsrequesting a traffic channel any one of said M traffic channels, ifavailable, for a duration of a call transaction, and to deallocate saidtraffic channel and to command said one mobile station to return ontosaid control channel after completion of said call transaction; meansfor collecting data on collisions of call transactions of differentcalls on said M traffic channels, a collision of call transactions beingdetected, if said call control unit, due to a lack of available trafficchannels, fails to immediately allocate any one of said M trafficchannels to any one of said N mobile stations requesting a trafficchannel; and means for automatically adjusting the allowed maximumnumber of simultaneous calls in accordance with the collected data.